Genomic and Transcriptomic Insights Into the Autotrophic Metabolism on H ₂  +  CO ₂ or CO of the Thermophilic Acetogenic Model Organism Moorella thermoacetica

Although the Wood-Ljungdahl pathway, a wide-spread pathway for CO₂ fixation in anaerobic microorganisms, was elucidated in the thermophilic acetogenic bacterium Moorella thermoacetica, still relatively little is known about the enzymes involved in hydrogen oxidation, CO₂ fixation, energy conservation and the role of quinones and cytochromes. Here, we have used transcriptomics, enzyme assays and genome analyses to identify missing links. NADPH, generated by a [FeFe] hydrogenase, is the reductant for CO₂ reduction to formate, a key reaction in CO₂ fixation. This reaction is slightly endergonic under standard conditions but becomes thermodynamically feasible at high environmental H₂ concentrations. In addition, formate is taken out of equilibrium by a formate dehydrogenase that potentially forms a complex with an energy-converting hydrogenase (Fdh-Ech), a novel respiratory enzyme in acetogens. Under low H₂ concentrations, the complex can drive the reverse, endergonic reaction. In addition, we postulate a formate cycle involving a periplasmic, cytochrome b-containing formate dehydrogenase. A NADH dehydrogenase-like enzyme that uses reduced ferredoxin instead of NADH to reduce menaquinone is also involved in energy conservation. The data are summarised in a comprehensive metabolic and bioenergetic model of acetogenesis from H₂ + CO₂ and CO in M. thermoacetica.